This invention relates to determining and adjusting the torque of an electric torque wrench, particularly an electric torque wrench employed for tightening pipe flanges.
While pneumatic impact torque wrenches have long been available, such wrenches require a source of pressurized air. Electric impact torque wrenches, therefore, are generally employed for field or other portable uses. For example and for particular purposes of the present invention, electric impact torque wrenches are advantageously employed by pipe-fitters for tightening the bolts of pipe flanges at a job site, during construction or repair of plumbing systems.
Pipe flanges, particularly pipe flanges for pipes carrying fluids under pressure, often employ gaskets therebetween to prevent leakage of the fluids out of the flanged joint. The gasket is resilient and functions by compressing, thereby conforming to surface irregularities of the flange surfaces when bolts between the pipe flanges are tightened. The gasket compresses particularly in regions proximate the bolts which are being tightened.
Because the gasket compresses and allows movement of the flanges toward one another while the bolts are being tightened, it is often important to tighten the bolts so as to distribute the tightening stresses around the pipe flanges as evenly as possible. This helps to draw the flanges together evenly, which reduces differential stresses on the flanges as well as causes the gasket to compress more evenly and, therefore, to more effectively prevent leakage.
Bolt tightening methods known in the art to be effective at distributing applied tightening stresses evenly over the flanges and gasket of a pipe joint typically employ a pattern of tightening the bolts wherein the pattern is repeated for a plurality of increased torque values of a tightening device such as a torque wrench. For example, where a pipe joint flange employs four bolts which, for reference, are labeled #1, #2, #3 and #4 as they appear in a clockwise pattern, a preferred tightening pattern might be to tighten bolt #1, followed by tightening bolt #3, followed by tightening bolt #2, followed by tightening bolt #4. All of the bolts would be tightened in a first round of tightening. Then the same tightening pattern would be employed for a second round of tightening. Typically, three rounds of the tightening pattern are employed to complete tightening of the pipe joint flanges.
Carrying out a preferred bolt tightening method for pipe joints, such as that aforedescribed, requires a device for applying a well controlled torque. Hand operated torque wrenches may provide for a relatively precise application of torque but have well known disadvantages. For example, the passive torque wrench is relatively slow and requires a greater span over which a handle end thereof is permitted to swing.
Electric torque wrenches may provide for greater speed and reduced operator work and space requirements, however, controlling the torque in such devices has required relatively expensive electrical or mechanical components. Typically, such devices measure the torque directly, by employing electro-mechanical sensors for force and, sometimes, angle of rotation. Such devices must also include relatively expensive measuring circuitry to interpret the output of the sensors. As a result of the cost of such devices, particularly for connecting pipe joints for plumbing construction or repair, use thereof is often foregone and hand operated tools are employed instead.
Where electric torque wrenches have been employed, it has nonetheless been difficult for operators to determine what the torque settings of the wrench should be. For one thing, the torque required for tightening a pipe joint depends on the physical size of the gasket or surface area thereof However, plumbers are used to thinking in terms of the diameter of the pipe. But the gasket surface area is not in general determined for a given pipe diameter. Rather, a single pipe diameter may employ flanges having a diameter that lies anywhere within a range.
Accordingly, there is a need for a method and apparatus for determining and adjusting the torque in an electric impact torque wrench which provides for relatively low cost control of torque, particularly for use in a standard tightening method for the tightening of pipe joints, and which provides for facilitating the determination of desirable torque settings by plumbers in the field.